NOx-storage catalytic converters are utilized in internal combustion engines which can be operated with a lean air/fuel mixture (lambda>1) in order to store the NOx emissions which are discharged by the engine during a lean operation. Here, the NOx-storage catalytic converter is in the so-called storing phase. The efficiency of the NOx-storage catalytic converter reduces with increasing duration of the storing phase, which leads to an increase of the NOx emissions rearward of the NOx-storage catalytic converter. The cause for this reduction of the efficiency is the increase of the nitrogen oxide (NOx) fill level of the NOx-storage catalytic converter. The NOx fill level can be monitored and a discharge phase or a regeneration phase of the NOx-storage catalytic converter can be initiated when a pregiven threshold value is exceeded. A nitrogen oxide NOx storage model can be utilized to determine the NOx fill level of the NOx-storage catalytic converter. NOx storage models are generally known from the state of the art. In an NOx storage model, the NOx fill level can be modeled from parameters which define the operating point of the engine. These parameters include, for example, the supplied fuel mass or air mass, the torque, et cetera.
During the discharge phase, a reducing agent is added to the exhaust gas of the internal combustion engine which reduces the stored nitrogen oxides to nitrogen (N) and carbon dioxide (CO2). Hydrocarbons (HC), carbon monoxide (CO) and/or hydrogen (H2) can be used as reducing agents. These reducing agents can be generated by a rich adjustment of the air/fuel mixture in the exhaust gas (homogeneous operation of the internal combustion engine). HC, CO and H2 are also characterized as rich gases. Alternatively, urea can be added to the exhaust gas as a reducing gas. Ammonia from the urea is used to reduce the nitrogen oxide to nitrogen and carbon dioxide. The ammonia can be obtained from a urea solution by hydrolysis.
Toward the end of the discharge phase, a large part of the stored nitrogen oxide is reduced and less and less of the reducing agent impinges on nitrogen oxide which it can reduce to nitrogen and carbon dioxide. As a consequence, toward the end of the discharge phase, the component of the reducing agent increases in the exhaust gas rearward of the NOx-storage catalytic converter; the component of oxygen in the exhaust gas rearward of the NOx-storage catalytic converter reduces. The end of the discharge phase can then be initiated from an analysis of the exhaust gas rearward of the NOx-storage catalytic converter by suitable exhaust-gas sensors (for example, O2 sensor or NOx sensor) when the greater part of the nitrogen oxide has been discharged from the NOx-storage catalytic converter. Furthermore, it is known to determine the NOx fill level of the NOx storage catalytic converter by means of a discharge model and to therewith determine, model-supported, the end of the discharge phase.
The end of the regeneration phase must be determined as accurately as possible because a regeneration phase which is too short does not completely empty the NOx-storage catalytic converter and, as a consequence thereof, the NOx emissions increase. On the other hand, a regeneration phase, which is too long, leads to an increase of the reducing agent emissions (rich gases or urea). An increase of the NOx emissions as well as an increase of the reducing agent emissions is harmful to the environment and should therefore be reduced to a minimum.
The use of suitable exhaust-gas sensors for analyzing the exhaust gas rearward of the NOx-storage catalytic converter and to fix the end of a regeneration phase is relatively complex and expensive. In the known model-supported methods for determining the end of the regeneration phase, a reducing agent mass flow is determined from a composition (lambda) of the air/fuel mixture and an air mass is supplied to the engine for combustion. This reducing agent mass flow is converted to a mass flow via a temperature-dependent factor in dependence upon which a reduction of the NOx, which is stored in the NOx-storage catalytic converter, is computed during the lean operation of the engine.
This modeling has the disadvantage that it is relatively inaccurate and is useful only to a certain extent to determine the end of a regeneration phase. This has its cause especially in that the reducing agent likewise reduces stored O2 in addition to the stored NOx during the regeneration phase. Which stored gas (NOx or O2) is actually reduced at a specific time point during the regeneration phase is dependent upon the type of construction of the NOx-storage catalytic converter. Accordingly, it cannot be determined from the discharge model known from the state of the art which gas at which time point and how much is reduced during the regeneration phase.